1. Statement of the Technical Field
The invention concerns radio communication method, apparatus, and system. More particularly, the invention concerns a method for a digital radio to provide an audible indicator of communication link quality.
2. Description of the Related Art
Analog radios provide a communication link for a baseband signal, such as an audio signal, by use of an analog radio transmitter and an analog radio receiver. An analog radio transmitter operates by amplifying the baseband signal, modulating the baseband signal by use of analog modulation techniques that are known in the art, frequency upconverting the modulated signal to radio frequencies (RF), and transmitting the RF signal to an analog radio receiver. The analog radio receiver recovers the baseband signal by downconverting and demodulating the received RF signal. Radio operators such as public safety personnel are very familiar with the operation of analog radios. Analog radios are simple, but a disadvantage of analog radio is that the quality of the received radio transmission, after demodulation in order to provide a received baseband signal, is prone to be poor (e.g., noisy) in situations such as low RF received power level, low signal-to-noise (SNR) ratio, and the presence of interference.
Receiver performance of analog radios gracefully degrades, such that the radio operator can hear increased noise on the demodulated baseband signal as the received RF signal weakens or the SNR degrades. The increased noise provides an aural cue to the radio operator, who may then use the aural cue to move to an area of better coverage. Furthermore, if the analog radio operator is able to hear conversations of other radio operators, the radio operator may be able to get additional audio cues of quality by listening to the quality of those other conversations on the analog radio.
In contrast, digital radios employ digital modulation techniques that are known in the art in order to provide a digitized communication signal from a transmitter to a receiver. The digitized communication signal may include a digitized baseband voice signal, or other baseband audible signals (e.g., music), or IP-based data traffic. Compared to analog radios, digital radios provide a relatively noise-free received demodulated baseband signal under typical operating conditions. Digital radios improve the received signal quality delivered to a radio operator over a wide range of received signal conditions by using error detection and correction techniques. Digital radios also provide other benefits compared to analog radios, e.g., more efficient spectral usage. The usable error detection and correction techniques may vary depending upon the type of communication and the latency, and may be implemented at different levels of a protocol stack.
At a link layer, error detection and correction techniques may include an error-correcting code (ECC). An example of an ECC is a forward error correction (FEC) code. The transmitter encodes the data with an error-correcting code (ECC) and sends the coded message. The receiver receives a noise-corrupted signal, and makes a maximum-likelihood estimation of the original transmitted message. ECC decoders are often located close to the front end of the digital radio receiver, e.g., in the first stage of digital processing after a signal has been received. ECC coders may also generate a bit-error rate (BER) signal or error count signal, which can be used as a feedback to gauge the quality of the received signal. The BER may be an uncoded BER, which is the bit error rate prior to ECC correction, or a coded BER which is the BER after ECC decoding and which is what is delivered to the listener. The uncoded BER is more useful than a coded BER for the purpose of monitoring RF link degradations, because the uncoded BER is more sensitive to such degradations.
An example of a digital radio is the Harris' OpenSky® family of products, which offers digital audio and packet data communications using a high performance IP backbone network. OpenSky uses a continuously transmitting base station with separate error correction schemes for control channel and data. Continuous monitoring of base station traffic can provide a received signal strength indicator (“RSSI”) and error numbers provided by an ECC decoder.
The error detection and correction capabilities of digital radio provide high-quality baseband analog audio transmission capability, as long as the digital radio is operating within the error detection and correction limits of the decoder. This is generally seen as an advantage because it increases the useful range within which digital radios can operate compared to analog radios.
However, beyond the error correction capabilities of the error detection and correction code, performance rapidly degrades. This presents a human-factors problem for operators of digital radio because the communication link appears to fail unexpectedly, without adequate warning to a radio operator. Furthermore, digital radios are frequently trunked—i.e., operated by packet transmission in order to deliver a communication only to an intended recipient—so that the radio operator is not able to receive an aural cue of transmission link quality by listening to other radio operators' communications.
Some radio operators (e.g., firefighters) object to the absence of an intuitive awareness that the signal is degrading and that interruption of communications is imminent. Some radio operators find this shortcoming objectionable enough to decide to revert to their familiar analog systems.